The present invention relates to methods and apparatus for the compaction of foundry molding material, more specially foundry sand, using a pressing plate acting directly on the surface of the molding material, and travelling during its working stroke with a speed of up to 20 meters per second.
During the course of the last few years the technology of compacting foundry molding materials has entered into a stage of significant development, the guiding star of such improvements having been more particularly the concept of making working conditions in the industry less arduous for the labor employed, and paying special attention to the quality of the air and to the noise level. To take an example, because of the amount of noise produced there has been a swing away from customary shaking and combined pressing and shaking in favor of pneumatically operating machines, as for example mold blasting machines, in which precompaction is able to take place by the deceleration of a pneumatically accelerated volume of molding material on the pattern and the pattern plate. In this case mechanical postcompaction is generally necessary in order to attain a sufficiently high degree of strength at the outline or contour of the mold.
Of late compacting methods operating purely pneumatically have been devised, in which after the molding material has been deposited in the flask, it is subjected to an impact-like pulse of gas under pressure, that is to say, with gas, which has been highly compressed by a compressor or the like or gas in the form of mixture such that it may be exploded. While it is true that such a method has made it feasible to produce a dramatic decrease in the costs of mold making as compared with conventional methods and the quality of the mold has been greatly improved, in the case of a large number of pattern forms unforeseen difficulties have been experienced in mold production from some patterns. On the other hand the new method does not make it possible to mold the center riser or the pouring basin directly, because the back of the mold remains relatively soft. In the case of such forms of materials as for example spheroidal graphite cast iron or cast steel, a hard mold back and, because of loads during casting, a high degree of hardness throughout is desired, this again making it necessary for the mold to be mechanically pressed in a further working step so that the said new method becomes excessively involved technically. Generally one may say that while such compaction methods do make it possble to reduce mold making costs, the practical applications have so far been limited.
Furthermore proposals have been made recently to drive pressing members such as pressing platens, diaphragms, pressing punches or the like by gas under pressure, but however such methods have not advanced to the stage of being practically important, most probably because the compacting effect has not been any better than that of conventional hydraulic or pneumatic pressing methods.
Lastly there has been a proposal (see "Litejnoe Proizvodstvo in Deutsch" 1963, no. 3, pages 6 to 9) to accelerate a plate freely resting on the molding material by a pulse. Such so-called "high velocity pressing" is made possible by igniting an explosive material to violently drive an impact piston in a cylinder so that the piston is rammed against a pressing plate and gives up its kinetic energy to the plate. Consequently the pressing plate is very abruptly accelerated up to its maximum speed and during the compaction stroke it is braked by the internal friction of the mold material particles and decelerated to zero velocity. The changes in the rate of deceleration as a function of time are largely dependent on the elastic properties of the pressing plate and the damping behavior of the mass of molding material. Fluctuations in the properties of the molding material as are likely in practice, and different heights of the mold material, necessary for different patterns, lead to different compacting effects, that are furthermore interfered with by superimposed impact waves. In order to avoid axial impact waves from coming into being, the propelling gas over the impact piston is allowed to expand to zero pressure through exhaust ports even before the impact on the pressing plate.
In writings on the subject it has furthermore been pointed out that spalling cracking of the back of the mold or even damage to the material of the mold are likely, apparently because the compression of the back of the mold is excessive owing to the high initial acceleration so that a mold is in danger of cracking when it is depressurized. This method would seem so far only to have been used on a laboratory scale, because of these shortcomings as noted and also because in the case of a normal overall size of the flask and a correspondingly large compaction stroke high explosives with a correspondingly high energy content had to be employed, which naturally constitute a safety risk. An advantage to be encountered with such dynamic presses is however the high degree of hardness of the mold attainable.